Mobile communication system now pays close attention to high-speed data services and user experiences. For example, both a Long Term Evolution (LTE) system corresponding to Evolved Universal Terrestrial Radio Access (E-UTRA) protocol specified by 3rd Generation Partnership Project (3GPP) and a LTE-Advanced system provide high-speed mobile data services. By means of Carrier Aggregation (CA) technology, the peak rate of the LTE-A system may exceed 1 Gbps. Moreover, through improving the architecture of the LTE system, the number of network nodes in the LTE-A system is reduced, and then end-to-end delay is improved. Accordingly, compared with the previous 3G systems, the LTE-A system may provide better user experiences.
FIGS. 1A and 1B are diagrams illustrating the structure of System Architecture Evolution (SAE) of 3GPP LTE system. Such flat and all-IP structure is convenient for obtaining data.
User Equipment (UE) 101 is a terminal device for receiving data.
Evolved Universal Terrestrial Radio Access Network (E-UTRAN) 102 is a radio access network which includes an evolved Node B (eNodeB/eNB) for providing a radio interface for the UE. eNodeBs are connected to each other through an X2 interface. The E-UTRAN is connected to an Evolved Packet Core (EPC) network through an S1 interface.
The EPC network is responsible for overall control of UE and bearer establishment. A Mobile Management Entity (MME) 103 is configured to manage mobility contexts, session contexts and security information of the UE. A Serving Gateway (SGW) 104 is configured to provide functions of User Plane (UP). The MME 103 and the SGW 104 may be in the same physical entity.
A Packet Gateway (PGW) 105 is configured to implement charging and legal monitoring functions. The PGW 105 and the SGW 104 may be in the same physical entity.
A Policy and Charging Rules Function (PCRF) 106 is configured to provide QoS policies and charging rules.
A Service GPRS supporting Node (SGSN) 108 is a network node device for providing routing for data transmission in a Universal Mobile Telecommunications System (UMTS).
A Home Subscriber Server (HSS) 109 is a home sub-system of the UE and is configured to protect user information including the current location of the UE, the address of a serving node, user security information and packet data contexts of the UE.
As well known, transmission capacity in a communication system is limited by bandwidth resources. Especially, radio spectrum used for mobile services becomes scarce increasingly, while mobile Internet is developed rapidly, and thus a mobile communication network has a growing need in providing a high-speed multimedia service for users. Accordingly, how to transmit massive user data with high efficiency for the purpose of relieving the load pressure of the radio access network, transport network and bearer network caused by ever-growing mobile traffic increase has become an urgent issue. For example, the load pressure may include the transmission pressure of LTE-Uu air interface of the radio access network, the transmission pressure of the backhaul link between the radio access network and a core network and the transmission pressure of the link between the radio access network and an operator network/Internet.
Conventionally, the physical capacity of network may be enlarged to address the increase of mobile traffic. For example, more spectrum can be added, Multi-input Multi-output (MIMO) transmission technology may be improved to increase spectrum efficiency, the number of base stations/cells may be increased, more micro-cells/small cells may be established and fiber capacity may be improved etc. However, these conventional solutions for solving the problem caused by the increase of mobile traffic all dramatically increase the establishment costs of operator networks, that is to say, probably revenue cannot scale with the increase of data volume. However, coupling the increasing of traffic volume with physical capacity increasing is simply formulated under the assumption that that data contents transmitted by different users are different.
However, recent analysis reveals that in massive data transmitted in the mobile communication network, many data contents hold a large number of users' common interest. For example, these interesting data contents may be popular multimedia data such as hit series, sports events and important news. If these data contents are transmitted on an air interface through a conventional point-to-point transmission mode, resources may be wasted. For example, a separate link is established between an eNodeB and a UE in the LTE system, and a unicast mode is adopted. In order to improve the efficiency of radio spectrum of air interface, a Multimedia Broadcast and Multicast Service (MBMS) may be used to transmit data in the LTE system. Multiple UEs may obtain data from an eNodeB through one link, which may reduce the transmission pressure of air interface to some degree. However, there is still a problem, that is, these users may need the data at different time points. Besides the transmission pressure of air interface, the increase of mobile traffic causes the transmission pressure of backhaul link, where data contents with high popularity and long life cycle are transmitted repeatedly between the radio access network and a network data center, thereby causing the unnecessary waste of link resources.
At present, study item of Device to Device (D2D) direct communication technology has been approved by 3GPP and is becoming a candidate evolution direction of the LTE-A system because of its great potential value in a public safety field and a general civilian communication field. During a D2D communication process, it is an important step to find a D2D terminal. Only if this step is performed, a communication initiator terminal having D2D communication requirements may find other D2D terminals within an effective distance. The effective distance refers to a coverage range of D2D communication capability of the communication initiator terminal. If there are one or more target terminals within the effective distance, the communication initiator terminal may establish a communication link with the one or more target terminals and perform D2D communication through a series of subsequent processes. At present, 3GPP has begun to specify a mutual discovery mechanism and a D2D communication mechanism with the assistance of the LTE system. If all D2D terminals capable of D2D communication within a certain range are referred to a D2D group, the D2D terminals in the D2D group may share data directly. When a D2D terminal has obtained and cached data holding many users' interest, other D2D terminals may obtain the data from the cache of the D2D terminal. In this way, the load pressure of network may be relieved.